Data from observational studies suggest that plasma total homocysteine (tHcy) concentration may be an independent and modifiable risk factor for cardiovascular disease (CVD) in adults. Plasma tHcy levels respond rapidly to nutrient supplementation with folic acid and vitamins Bl2 and B6, alone or in combination. The available data and the potential for prevention provide a strong rationale for understanding determinants of tHcy in youth. However, no large U.S. studies have examined the relation between tHcy levels to individuals' dietary intakes of folic acid and vitamins Bl2 and B6 in youth. This application for a competing continuation of the Child and Adolescent Trial for Cardiovascular Health (CATCH Phase 3, 1994-1997) presents a unique opportunity to examine the relation between diet and serum tHcy levels in an ethnically and geographically diverse cohort of adolescents (n=3524). During CATCH Phase 3, we conducted a cross-sectional study of serum tHcy levels in eighth grade students (Jan-June 1997). We observed higher mean tHcy levels among males, Blacks, and non-users of multi-vitamins and a strong, inverse association with serum levels of folic acid and to a lesser extent, with serum vitamin Bl2. Our aim is to conduct a second measurement of the cohort at grade 12 in order to assess the dose-response relation between serum tHcy and dietary intakes (not measured in grade 8) of folio acid, vitamin B12 and vitamin B6. In addition and of equal importance, we aim to evaluate changes in serum folio acid and tHcy levels from grade 8 to 12, after full implementation of fortification of cereal and grain products in the U.S. with folio acid and describe its impact on the distribution of tHcy levels in adolescents. Effective January 1, 1998, the U.S. Department of Agriculture mandated the addition of folic acid to all flour and grain products in the United States (FDA, Federal Register, 1996). CATCH has a unique opportunity to examine the effect of "this natural experiment" on the distribution of serum tHcy in the cohort, because serum tHcy levels were measured in grade 8, just prior to full implementation of the mandate. Furthermore, the proposed study will have adequate sample size to examine these changes among important clemographic-subgroups such as, males vs. females and Caucasians vs. African Americans vs. Hispanics. Information generated by this study will be valuable for designing specific dietary interventions for youth and targeting subgroups of children who may be at higher risk for CVD.